Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing my new design.

The first broken line rounded rectangle outside the solid line rectangle illustrates a display screen and forms no part of the claimed design. The broken lines outside the display screen show a device and form no part of the claimed design. The broken lines inside the display screen show portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 